A polymer electrolyte fuel cell (or a solid polymer fuel cell) comprises a solid polymer electrolytic membrane composed of an ion exchange membrane (a polymer membrane having ion conductivity) such as perfluorocarbonsulfonic acid in which a sulfonic acid group is introduced into a fluorocarbon skeleton (such as a polytetrafluoroethylene skeleton), two electrodes disposed on both sides of the electrolytic membrane, separators with grooves for supplying a gas such as hydrogen gas and oxygen gas to each electrode, and two current collectors disposed on both sides of these separators.
Among these constituting members, the separator is particularly required to have a low electrical resistance (electrical conductivity) in addition to such properties as gas impermeability, stability for warm (hot) water, sulfuric acid resistance and a high mechanical strength. Traditionally, there has been studied a method for shaping a plate material made of titanium or graphite by a mechanical processing such as a machining processing. However, this process lacks mass-productivity and it is difficult to carry out the process on an industrial scale.
Japanese Patent Application Laid-Open No. 334927/1998 (JP-10-334927A) discloses a separator for a polymer electrolyte fuel cell, which is obtained by molding a resin composition comprising a carbon powder and a thermosetting resin (e.g., a phenolic resin, a polyimide resin, an epoxy resin, and a furan resin) by means of a resin molding method (or process). However, the phenolic resin used as the thermosetting resin is slow in hardening, and low in productivity. For example, in Examples of the document, it takes not shorter than ten hours to harden the resin. Moreover, since a gas such as water vapor is generated as accompanied with hardening the phenolic resin, the hardened product has warp and is deteriorated in gas impermeability.
Japanese Patent Application Laid-Open No. 267062/1992 (JP-4-267062A) discloses a gas separator for a fuel cell, which is made of stainless steel or copper. However, although a high industrial productivity is achieved by making the separator with the above metal, the deterioration of the material is caused by contacting the separator with fuel hydrogen gas for a long period of time. Accordingly, the separator rapidly deteriorates cell characteristics.
Further, Japanese Patent Application Laid-Open No. 151833/2001 (JP-2001-151833A) discloses a hardening resin composition consisting of (A) a vinyl ester resin, (B) a monomer selected from the group consisting of an allyl ester monomer, an acrylic ester monomer and a methacrylic ester monomer, (C) a radical polymerization initiator and (D) a carbon-series filler. This document describes that the resin composition has a conducting property and a low electric resistance, and is particularly useful as a separator for a fuel cell. Japanese Patent Application Laid-Open No. 164063/2002 (JP-2002-164063A) discloses a resin composition for a fuel cell, which comprises an electroconductive agent, a radical-polymerizable resin (e.g., a vinyl ester-series resin), and a radical-polymerizable diluent. This document mentions that the acid value of the radical-polymerizable resin is 0.1 to 5 mgKOH/g. However, this resin composition is apt to deteriorate in electroconductivity, and it is difficult to impart a high electroconductivity to the resin composition while maintaining sufficient mechanical strength.
Further, U.S. Pat. No. 6,251,308 specification discloses a molding composition comprising a radical-polymerizable resin such as an unsaturated polyester resin or a vinyl ester resin, a polymerizable unsaturated monomer having an end ethylene group, an electroconductive filler, a polymerization initiator, and a rheological agent. However, since the rheological agent [or a rheological modifier, so-called a thickening agent] is essential for the molding composition, it is difficult to obtain a molded article (e.g., a plate for a fuel cell) having enough long-term reliability [for example, stability for hot water (e.g., a retention of mechanical property or a retention rate of weight) after treating the molded article in 90° C. hot water for 2000 hours, and chemical resistance (such as acid resistance)]. Moreover, in a kneading method involving heat generation (e.g., a kneading method using a pressure kneader), the composition containing the rheological agent rapidly increases in viscosity during kneading in many cases. As a result, it is difficult to obtain, from the composition, an improved compounded matter (or an improved compound) excellent in moldability.
It is therefore an object of the present invention to provide an electroconductive resin composition suitable for an electroconductive plate [for example, a separator for a fuel cell (in particular, a polymer electrolyte fuel cell)], an electroconductive plate (e.g., the above-mentioned separator) formed from the resin composition, and a process for producing the electroconductive plate (e.g., the above-mentioned separator) advantageously on an industrial scale.
It is another object of the present invention to provide an electroconductive resin composition having a low electric resistance and showing an improved gas impermeability, stability for warm (hot) water, durability (in particular, acid resistance such as sulfuric acid resistance) and a high mechanical strength, an electroconductive plate (e.g., the above-mentioned separator) formed from the resin composition, and a process for producing the electroconductive plate (e.g., the above-mentioned separator) conveniently and efficiently.
It is still another object of the present invention to provide an electroconductive resin composition with high dimensional accuracy, and a process for producing an electroconductive plate (in particular, a separator for a fuel cell) from the resin composition with high molding accuracy.